Stellar collisions in galactic centres: black hole growth and colour gradients

Abstract

We study the effects of stellar collisions, particularly on feeding massive black holes (BHs) and colour gradients, in realistic galactic centres. We find that the mass released by stellar collisions is not sufficient to account for the present BH mass in galactic centres, especially in bright galaxies. This study, together with the study by Magorrian & Tremaine on tidal disruption of stars by massive BHs, implies that the material for BH growth (especially in galaxies brighter than ~10⁹ L_solar) can only come from other sources, for example, the mass released by stellar evolution in the initial ~1 Gyr of the lifetime of the galaxy, or the gas that sinks to the galactic centre in a galaxy merger. We also analyse how the colour of a stellar system is affected by collisions of stars. We find that collisions between main-sequence stars cannot cause observable colour gradients in the visible bands at projected radius R>~ 0.1 arcsec in M31, M32 and other nearby galactic centres. This result is consistent with the lack of an observable colour gradient in M32 at R >~ 0.1 arcsec. At even smaller radii, the colour differences caused by collisions between main-sequence stars are at most 0.08 mag at R = 0.02 arcsec. The averaged bluing caused by stellar collisions in the region R < 0.1 arcsec of M32 should not be larger than 0.06 mag in colour index U - V and 0.02 mag in V - I. The observed bluing in the centre of the galaxy M31 (in a 0.14 × 0.14 arcsec² box) must be caused by some mechanism other than collisions between main-sequence stars.